Apparatus for forming a phase hologram on a deformable thermoplastic

ABSTRACT

Method and apparatus for producing holographic interference patterns wherein a modulated coherent object beam and off axis reference beam cooperate to discharge a photoconductive thermoplastic recording member which is then recharged and allowed to deform in accordance with the residual charge pattern thereon. The off axis angle is a function of the quasi-resonant frequency of the thermoplastic.

7 zez ll i United Stal Urbach [541 APPARATUS FOR FORMING A PHASEHOLOGRAM ON A DEF ORMABLE THERMOPLASTIC [72] Inventor: John C. Urbach,Penfield, NY. 73] Assignee: Xerox Corporation, Rochester, NY. [22]Filed: Oct. 6, 1970 [21] Appl. No.: 78,536

Related US. Application Data [62] Division of Ser. No. 521,982, Jan, 20,1966, Pat. No.

[52] US. Cl ..350/3.5, 346/77 E [51] Int. Cl. ..G02b 27/00 [58] FieldofSearch ..350/3.5, 162 SF; 340/173 TP;

[56} References Cited UNITED STATES PATENTS 3,506,327 4/1970 Leith et al"35013.5

II I.)

[151 3,655,257 [451 Apr. 11, 1972 3,436,216 4/1969 Urbach ..96/l.l

OTHER PUBLICATIONS Cathey, low. of the Optical Society of America, Vol.55, April 1965, p. 457

Primary Examiner-David Schonberg Assistant Examiner-Ronald J. SternAttorney-Frank A. Steinhilper, Stanley Z. Cole and James J. Ralabate[57] ABSTRACT lClaimsADrawlngHgum Patented Aprilll, 1972 INVENTOR '40 c..uagAcl-l APTORNEY APPARATUS F OR FORMING A PHASE HOLOGRAM ON A DEFORMABLE THERMOPLASTIC This is a division of application Ser. No.521,982, filed in the United States, Jan. 20, 1966, now US. Pat. No.3,560,205.

This invention relates in general to imaging systems, and, morespecifically, to a unique holographic imaging system.

Holography as initially described by Dennis Gabor in an articleentitled, A New Microscopic Principle appearing in Nature 161, 777-778(1948) is a two-step imaging process in which the diffraction pattern ofan object illuminated with coherent radiation such as light is recordedon a radiation sensitive layer. This record, known as a hologram, isthen used to reconstruct an image forming wavefront by reillumina tingthe hologram with coherent electromagnetic radiation. One of theimportant problems which existed in holography as originally developedwas that reconstruction of the image was imperfect due principally tothe fact that the in-focus image had a defocused conjugate or twin imageas well as non-infonnation carrying light as a background owing to thefact that the phase portion of the original coherent beam wasambiguously recorded. This defect has recently been overcome by amodification of the holographic system known as the off axis referencebeam technique described by Leith, E. and Upatnieks, J. in two articlesin the Journal of the Optical Society of America; ReconstructedWave-Fronts in Communication Theory" 52, 1,123 October, 1962 andWave-Front Reconstruction with Continuous Tone Objects, 53, 1,377December, 1963. In this off axis reference beam technique a referencebeam of coherent light is brought in at an angle with respect to thebeam of coherent light used to expose the image, thus forming aninterference pattern on the photosensitive recording plate. Afterdevelopment, this plate or hologram is again exposed to coherentradiation and the image is reconstructed off the optical axis at anangle proportional to the angle of incidence of the original referencebeam. The desired reconstructed image is thereby separated from the outof focus twin image as well as from the beams which do not carry usefulinformation. In this technique then, the carrier wave detennined by thereference beam has a non-zero mean frequency. The carrier wave ismodulated by the object waveform as a result of the amplitude squaringprocess that takes place during recording of the hologram.

Holography has a great many advantages over other imaging techniques,for example, a hologram may be used to reconstruct a three dimenstionalimage with radiation of another wavelength than was used to record thehologram, resulting in extremely high magnification. It may be used forthe secure transmission of information (since the hologram itself bearslittle or no resemblance to the original image) and may also be used forrapidly making many images of the original since when the hologram iscut into a number of sections, each section contains all of theholographic information necessary to reconstruct the original image.

Although most experimenters have worked with amplitude holograms, it isalso possible to make phase holograms in which the image information isstored in a phase pattern rather than an intensity pattern as described,for example, in an article by G. L. Rogers in the Preceedings of theRoyal Society Edinburgh A-63, 193 (1953) and an article by W. T. CatheyJr., in the Journal of the Optical Society of America, 55, 457, (1965).Such phase holograms have previously been made from conventional silverholograms by bleaching the silver using the phase differences introducedby gell swelling and/or refractive index changes. Although these havecertain advantages over intensity holograms, they require extremelydifficult and complex processing to produce.

Holography also has a number of drawbacks. First of all, the silverhalide photographic recording plates used to form the holograms requirethe use of expensive high resolution material. In addition, the processis a relatively slow two-step affair in which reconstruction cannot becarried out until after the hologram has gone through the conventionalsilver halide photographic developing steps. Furthermore, no matter howhigh the resolution of the photographic film employed, the

silver halide grains in the emulsion are, by their very nature, discreteparticles which scatter light introduced noise into the optical systemin intensity holograms. Another problem encountered is that since thephotographic film is removed from the optical system during processing,the reconstructed image cannot be viewed as development takes place soas to optimize development of the hologram itself. Furthennore, theholographic recording material is not reusable to make differentholograms at a later time.

Accordingly, it is an objective of this invention to provide a novelholographic system devoid of the aforementioned deficrencres.

It is a further object of the invention to provide a novel holographicimaging method.

Still another object of the invention is to provide a novel holographicrecording material.

A still further object of the invention is to provide a holographicrecording system in which the optical system is tuned" to thecharacteristics of the recording material.

Yet another object of the invention is to provide a holographic systemwith almost instantaneous development which can be achieved withoutremoving the recording material from the optical system.

A still further object of the invention is to provide a holographicsystem with maximum sensitivity and minimum random noise.

The above and still further objects may be accomplished in accordancewith the present invention by recording a phase hologram on athermoplastic which will spontaneously deform upon softening when it issubjected to an applied electrical charge pattern. This is accomplishedby fonning a charge pattern on the thermoplastic which corresponds withthe optical interference pattern to be recorded as the phase hologramand then softening the thermoplastic until it deforms in confonnity withthis pattern. Although the off axis reference beam technique is apreferred method for forming the interference pattern to be recorded onthe thermoplastic, any suitable optical system capable of yielding thephase hologram construction exposure may be employed. lt is alsopreferred to tune" the hologram producing set-up so that the holographiccarrier wave is at or close to the quasi-resonant spatial frequency ofthe thermoplastic layer, as this produces an optimum low noise responsein the layer. As a consequence of the hydrodyanamic behavior of thisthin charged fiuid layer, the deformation of the thermoplastic whensoftened proceeds most rapidly when the electrostatic charge patter onit is at a particular spatial frequency which is here referred to as thequasi-resonant frequency. As explained in greater detail hereinafter,this quasi-resonant frequency is determined mainly by the thickness andto a lesser extent by the applied voltage on the thermoplastic. lnaddition, recording at or near this quasi-resonant frequency tends tosuppress random deformation which constitutes a source of noise in aphase hologram. Tuning of the system may be accomplished by changing theangle of incidence of the off-axis reference beam so that theinterfering wave-fronts of the reference beam and the object beam crossat an angle such that the interference fringes have a spatial frequency(reciprocal period) corresponding to the quasi-resonant frequency of theparticular thermoplastic being used. On the other hand, the angle of thereference beam in the holographic recording system may be retained at aconstant value while the thickness of the thennoplastic is changed tothereby change its quasi-resonant frequency to correspond with the anglebetween the object and reference beam.

The nature of the invention will be more easily understood when it isconsidered in conjunction with the accompanying drawings of an exemplarypreferred embodiment of the invention wherein:

FIG. 1 is a partially sectioned side view of an off axis reference beamholographic imaging set-up,

FIG. 2 is a side sectional view of the intersecting wavefronts from themain beam and the reference beam,

FIG. 3 is a side sectional view of one embodiment of a thermoplasticrecording beam according to the invention,

FIG. 4 is a side sectional view of a second embodiment of athermoplastic recording medium for use in the invention.

Referring now to FIG. I there is seen a Laser 11 coupled to a powersource 12. The Laser 11 serves as a source of coherent light for thesystem, however, it is to be understood that any other source ofsufficiently coherent light or other suitable electromagnetic radiationsuch as a coherent electron beam, X-ray beam, or the like may also beemployed. A portion of the coherent light 13 produced by Laser 11 passesthrough a partially transparent object 14 and impinges on the recordingplate generally designated 16. The remaining portion of the light beam13 is reflected off two mirrors l7 and 18 so that it also strikes therecording plate 16 but at an angle theta with respect to the undeviatedbeam. Although a transparent object is shown, it is of course to beunderstood that an opaque object may be employed with the light beambeing reflected off it and onto the recording plate so long as thereference beam also strikes the plate at the desired angle theta withrespect to the main beam. The processing steps in forming thedeformation image on the thermoplastic included in the photographicrecording member 16 will, of course, differ with the configuration ofthe particular recording member as described more fully hereinafter.This procedure may, for example, involve electrostatically charging therecording member, exposing it to the reference beam and the main beamthrough the object, recharging the thermoplastic recording member andsoftening it either by subjecting it to mild heating or solvent vaporsto cause the deformation pattern to appear on it. A unique feature ofthe invention is that the reconstructed image may be viewed while thethermoplastic imaging member is being developed. This is accomplished byremoving the object 14 from the optical path, blocking the referencebeam optical path and viewing the reconstructed image off axis at theangle theta from the main beam such as at point 19. Alternatively, themain beam may be blocked and the former reference beam used for formingthe reconstructed image. In this case, the true image will coincide inposition with the original object. Thus, the hologram recording plate isreconstructed during development by merely applying heat or solventvapor to the thermoplastic while viewing it off axis at angle theta.

FIG. 2 is a diagrammatic representation of how the wavefronts of themain beam and the reference beam cross and interfere with each otherwith wave-fronts 21 and 22 representing the wave-fronts from the offaxis reference beam and 23 representing the wave front from the mainbeam as modulated by the object. As can be seen by studying FIG. 2, thedistance between the two intersection points 24 and 26 can be altered bychanging the angle between the reference and the main beams with thedistance increasing towards infinity as the angle theta approaches 0.Preferably, the angle is adjusted so that the spacing is at or close tothe reciprocal of the quasiresonant frequency of the thermoplasticwithin about plus or minus l percent of the peak. Generally, this willrun about twice the thickness of the thermoplastic material, rangingfrom about 1.5 to about 2.7 times the thickness for films thicker thanabout 2 microns and from about 1.5 to about times this thickness forthinner films as described more fully in my copending applicationentitled Image Storage Ser. No. 476,533, filed Aug. 2, 1965.

In FIG. 3, there is shown the cross section of one exemplary embodimentof a suitable recording member for use in the invention. This recordingmember generally designated 16 may be in the form of a web, a rigidplate, a flexible endless belt or any other suitable mechanicalconfiguration. Although not entirely necessary, the recording membergenerally designated 16 includes an electrically conductive substrate soas to facilitate charging prior to an imagewise discharge of therecording member upon exposure. Suitable substrates include flexiblemetal foil or plates made of materials such as aluminum, brass, copper,etc., as well as fairly heat resistant polymers such as polyethyleneterephthalate, polycarbonates, polyurethanes and the like coated withthin transparent conductive layers of tin oxide, copper iodide, or thelike. With certain charging techniques such as two-sided coronadischarge where corona discharge of positive polarity is applied to oneside of the recording member while negative polarity corona discharge isapplied to the other, the conductive substrate may be eliminated fromthis system; however, it is generally desirable to incorporate such asubstrate to provide mechanical strength in the overall recordingmember. Over the substrate is a photoconductive insulating layer 28 anda deformable insulating thermoplastic layer 29. The photoconductivelayer 28 may consist of any suitable photoconductive insulating materialsuch as amorphous selenium or photoconductive pigments such as cadmiumsulfide, cadmium selenide, zinc sulfide, zinc selenide, zinc oxide, leadoxide, lead sulfide, mercuric sulfide, antimony sulfide, mercuric oxide,indium trisulfide, titanium dioxide, arsenic sulfide, galliumtriselenide, lead iodide, lead selenide, lead telluride, galliumtelluride, mercuric selluride, and the iodide sulfide selenide andtellurides of bismuth, aluminum, and molybdenum dispersed in aninsulating film-forming binder such as a silicone resin, a styrenebutadiene resin or the like. Other typical photoconductors include theorganics especially when these are complexed with small amounts ofsuitable Lewis Acids. Typical of these organic photoconductors are 1,4-dicyano-naphthalene; anthracene 3-benzylidene amino carbazole,2,5-bis-(p-aminophenyl-l )-l ,3,4-oxidiazole; vinyl carbazole;2,4-diphenyl-quinazolin; l-methyl-2-( 3,4-dihydroxymethylene-phenol)-benzimidazole and substituted andunsubstituted phthalocyanines and quinacridones in solutions ordispersed in insulating film forming binders of the type describedabove. Any suitable deformable insulating thermoplastic layer may beused as layer 29. Typical insulating thennoplastics include the glyceroland pentaerythritol esters of partially hydrogenated rosin;polyalphamethyl styrene, terpolymers of styrene, indene and isoprene;Piccolyte 5-70 and Sl00 (polyterpene resins made from beta-pineneavailable from Pennsylvania Industrial Chemical Co. and having ring andball melting points of 70 and 100 C., respectively); Piccopale 70 SF andPiccopale (non-reactive olefin-diene resins available from PennsylvaniaIndustrial Chemical Co., having melting points of 70 C. and 85 C. andmolecular weights of 800 and 1,000, respectively); Piccodiene 2,212 (astyrene butadiene resin available from the same company; PiccolasticA-75, D-IOO and 13-100, polystyrene resins with melting points of 75,and 100 C., respectively, available from the same company); CoumaroneIndene Resins available under the tradenames Neville R-2l and NevillacHard; Amberol ST-l37X (an unreactive, unmodified phenol formaldehyderesin available from the Rohm and Haas Chemical Company of Philadelphia,Pennsylvania; Aroclor 1,242, a chlorinated polyphenyl resin); PlioliteAC (a styrene acrylate copolymer); Pliolite VTAC (a vinyltoluene-acrylate copolymer); Neolyn 23 (an alkyd resin available froml-lercules Powder Co.) and mixtures of silicone and styrene resins. Inaddition, the thermoplastic insulating layer itself may bephotoconductive as shown by layer 30 in FIG. 4 and this may beaccomplished by taking any suitable photoconductive material anddispersing it, mixing it in solid solution, or copolymerizing it withthe resin material to form a single layer upon which the recording is totake place. In another approach, a thermoplastic insulating polymer ofthis type may be blended with a complexing agent to make itphotoconductive by forming a photoconductive charge transfer complex.Thus, for example, phenol formaldehyde polymer may be madephotoconductive by complexing it with 2,4,7- trinitrofluorenone or anyother suitable Lewis acid. In still a third embodiment of the recordingmember not shown, electrodes may be provided on both sides of a sandwichmade up of a photoconductive insulating layer and a deformableinsulating thermoplastic so that the required charge may be appliedthrough the electrodes rather than by corona discharge or some otherform of ionizing discharge as the type used with the recording webs ofFIGS. 3 and 4.

In imaging the recording web of FIG. 3, the thermoplastic insulatingsurface 29 of the web is first charged by grounding the conductivebacking 27 and passing it under a corona generating electrode connectedto a source of high potential adapted to uniformly charge this web. Thistype of corona charging technique is more fully described in US. Pat.Nos. 2,588,699 to Carlson and 2,836,725 to Vyverberg. However, it is tobe understood that any other suitable charging method may be used. Oncereading member 16 has been uniformly charged, it is placed into theoptical system of FIG. 1 and exposed as explained above, in the absenceof ambient light. If the recording member of FIG. 3 is then recharged inthe dark to a unifonn potential, a higher level of charge is built up onexposed areas of the recording web, owing to the movement of chargethrough the photoconductive layer 28 during exposure in the opticalsystem. This charge is then trapped on opposite sides of thethermoplastic layer 29 so that the recording web may even be handled inthe light after the recharging step has been carried out. In fact, afterrecharging is complete, the recording web may be replaced in the opticalsystem and again exposed to the main beam after the object 14 has beenremoved so that the hologram will be reconstructed simultaneously withits formation. In other words, one can watch the image form by viewingthe reconstruction from point 19. Fonnation of the hologram may beaccomplished by heating the thermoplastic surface which now carries acharge conforming to the image pattern with hot air or radiant heat orany other suitable heat source or by subjecting it to a solvent vapor orsome other suitable softening influence. Ripples then appear in thesurface of the thermoplastic owing to the effect of the charge patternon the softened material. This deformation technique is more fullydescribed in the recent literature. See, for example, an articleentitled, A Cyclic Xerographic Method Based on Frost Deformation," by R.W. Gundlach and C. .l. Claus, appearing in the January-February 1963issue of the Journal of Photographic Science and Engineering, and US.Pat. Nos. 3,ll3,l79; 3,196,011 and 3,196,008. When a recording member ofthe type shown in FIG. 4 is employed, softening of the photoconductivethermoplastic recording layer 30 is carried out in the dark so that theimagewise charge pattern is not dissipated.

It is, of course, to be understood that an apparatus for carrying outthe processing steps described above automatically is contemplated bythe invention. This type of apparatus may, for example, be arranged soas to charge, expose, recharge and soften the recording member withoutmoving it from its position in the holographic apparatus. Wheninspection development is used, a shutter is employed to close offeither the direct or the reference beam and the other beam is employedfor watching the reconstruction. The charging device is, for example,mounted in the machine so that it can be caused to scan across a surfaceof the imaging member while the heating device may be arranged toradiate heat towards the imaging member at selected times in theprocessing cycle.

The holograms of the invention have many advantages, one being that theymay be rapidly replicated by mechanical pressing using an intermediatemaster and is capable of reproducing very fine detail in the replicas.Thus, not only can the holograms themselves be produced very rapidly,but copies of these can also be produced more rapidly than would bepossible with convenient silver.

The following illustrative example of a preferred embodiment of theinvention is now given to enable those skilled in the art to moreclearly understand and practice the invention desirable results.

EXAMPLE An imaging member is first prepared by applying a coatingsolution to a N ESA glass substrate which is merely a glass base coatedwith an extremely thin layer of optically transparent tin oxide(available commercially from the Pittsburg Plate Glass Company ofPittsburg, Pennsylvania). This first coating comprises a solution of 20grams of polyvinylcarbazole and 0.1 grams of brilliant green crystals(CI. 42042) in l 10 grams of dioxane and l 10 grams of dichloromethane.It is applied so as to produce a photoconductive layer about 7 micronsthick. Over this layer, there is then applied a layer of StaybeliteEster 10 (a glycerol ester of 50 percent hydrogenated rosin availablefrom the Hercules Powder Company) to a dry thickness of about 0.6 micronapplied by withdrawing the substrate from a 20 percent solution of theStaybelite in a kerosene solvent at a rate of about 5 inches per minute.After drying, the completed imaging member consisting of a conductivebase, a photoconductive layer, and a deformable thermoplastic layer isinserted into an imaging system of the type shown in FIG. 1 and coronacharged in darkness to about 500 volts. It is then exposed to the objectand reference beams using a helium neon continuous wave laser operatingin the Tem 00 mode at 6328 ang strorn units (Model 5200 available fromthe Perkin-Elmer Company) using the same type of exposure setup shown inFIG. 1. The reference beam is brought in at an angle of about 304. Afterexposure the imaging member is recharged to 500 volts while stillremaining in the dark and the hologram is formed by mild heating of theStaybelite. This is accomplished by replacing the imaging member in theholographic setup, removing the object from the optical path andblocking off the reference beam while exposing the imaging member to themain beam and then viewing the imaging member at about 304 off axiswhile applying mild heat to its surface. The reconstructed image isviewed simultaneously with the formation of the hologram. A good qualityimage with resolution of 800 lines per millimeter is thus produced.

What is claimed is:

1. A holographic imaging apparatus comprising a source of coherentelectromagnetic radiation positioned to illuminate an object, a chargedeformable thermoplastic imaging member in the optical path of the beamemanating from said radiation source beyond said object, and means toproduce a second off axis coherent reference beam of electromagneticradiation positioned to intersect with id first mentioned beam ofcoherent radiation at the point 'n the optical path of saidfirst-mentioned beam where said" deformable thermoplastic imaging memberis placed, said imaging member further including a photgggndugtjve inylat o; whereby the interference pattern produced by said two coherentbeams is recorded in the form of a pattern of electrostatic charge onsaid imaging member and means to soften said thermoplastic whereby adeformation pattern corresponding to said charge pattern will formthereon, the angle of intersection between said first mentioned beam ofcoherent radiation and said reference beam of coherent radiation beingadjusted so that the spacing between adjacent carrier interferencefringes is within the range of plus or minus l5 per cent of thereciprocal of the peak of the quasi-resonant frequency of saiddeformable thermoplastic.

1. A holographic imaging apparatus comprising a source of coherentelectromagnetic radiation positioned to illuminate an object, a chargedeformable thermoplastic imaging member in the optical path of the beamemanating from said radiation source beyond said object, and means toproduce a second off axis coherent reference beam of electromagneticradiation positioned to intersect with said first mentioned beam ofcoherent radiation at the point in the optical path of saidfirst-mentioned beam where said deformable thermoplastic imaging memberis placed, said imaging member further including a photoconductiveinsulator whereby the interference pattern produced by said two coherentbeams is recorded in the form of a pattern of electrostatic charge onsaid imaging member and means to soften said thermoplastic whereby adeformation pattern corresponding to said charge pattern will formthereon, the angle of intersection between said first mentioned beam ofcoherent radiation and said reference beam of coherent radiation beingadjusted so that the spacing between adjacent carrier interferencefringes is within the range of plus or minus 15 per cent of thereciprocal of the peak of the quasi-resonant frequency of saiddeformable thermoplastic.